Calcium-rich melt inclusions in Cr-spinels from Borgarhraun, northern Iceland

Two groups of primitive melts are observed as inclusions in Cr-spinels in a picrite (Fo88–92.4) from northern Iceland. Group I inclusions have 12.2–13.8 wt% MgO, high CaO (14.6–15.8 wt%), low Al2O3 (10.1–11.8 wt%) and very high CaO/Al2O3 (1.3–1.5); they occur in spinels with a cr# between 0.63 and 0.70. Group II inclusions occur in spinels with slightly lower cr# (0.55–0.60) and they have 13.3–15.8 wt% MgO, 12.0–13.8 wt% CaO, 11.2–13.4 wt% Al2O3 and CaO/Al2O3 0.9–1.1. Both groups have low concentrations of the incompatible minor elements (TiO2, K2O and P2O5) and relatively high SiO2. The two groups converge at MgO approximately 10 wt%. A single inclusion in a low cr# spinel could represent a more evolved member (9.00 wt% MgO) of either group. Group I inclusions are on a clinopyroxene control line while Group II inclusions are close to being on an olivine control line. Groups I and II cannot be derived one from the other. They require high degree of melting from a refractory mantle source which probably is pyroxenitic to account for the high CaO. Melts with such high CaO/Al2O3 cannot be generated by melting commonly assumed lherzolites, so that pyroxenite or wehrlite mantle component is the suggested source.